The researchers said that as daily sitting time and TV-viewing time capture similar aspects of sedentary behavior, they "expected broadly similar magnitudes of associations from both exposures. Yet the effect of TV viewing on all-cause mortality seemed to be stronger in magnitude."
For TV-viewing time, the results differed in that they showed that high physical activity reduced, but did not eliminate, the mortality risks seen among those viewing TV for five hours or more a day.
The researchers believe that TV watching rather than just sitting down produced different results because of the dietary habits associated with TV watching, such as snacking or having a meal, which could affect people's metabolism.
"The effect of TV-viewing on all-cause mortality seemed to be stronger in magnitude. This difference is congruent with previous observations and might be partly due to differences in the accuracy of reporting these behaviours," the researchers noted.
"However, other explanations are also plausible. TV-viewing typically occurs in the evenings (at least, for the generation represented in the included studies), usually after dinner, and prolonged postprandial sedentary time may be particularly detrimental for glucose and lipid metabolism. It is also plausible that individuals break up their sitting time more frequently during work than when viewing TV, and breaking up sedentary time seems to be beneficial for various cardio-metabolic risk factors.
"Another explanation for the difference observed could be that TV-viewing might be accompanied by snacking behaviours and food advertising on TV might affect eating behaviour. Thus, associated dietary behaviours may explain some of the differences observed."
The researchers said their research results provided "further evidence on the benefits of physical activity, particularly in societies where increasing numbers of people have to sit for long hours for work or transport."
"Our findings indicate that increased sitting time is associated with increased all-cause mortality; however, the magnitude of increased risk with increased sitting time is mitigated in physically active people. Indeed, those belonging to the most active quartile and who are active about 60–75 min per day of moderate intensity physical activity seem to have no increased risk of mortality, even if they sit for more than 8 hours a day."
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